diff --git a/src/Composer/Json/JsonFile.php b/src/Composer/Json/JsonFile.php
index 1b20af274..458265e3e 100644
--- a/src/Composer/Json/JsonFile.php
+++ b/src/Composer/Json/JsonFile.php
@@ -18,16 +18,6 @@ use JsonSchema\Validator;
 use Seld\JsonLint\JsonParser;
 use Composer\Util\StreamContextFactory;
 
-if (!defined('JSON_UNESCAPED_SLASHES')) {
-    define('JSON_UNESCAPED_SLASHES', 64);
-}
-if (!defined('JSON_PRETTY_PRINT')) {
-    define('JSON_PRETTY_PRINT', 128);
-}
-if (!defined('JSON_UNESCAPED_UNICODE')) {
-    define('JSON_UNESCAPED_UNICODE', 256);
-}
-
 /**
  * Reads/writes json files.
  *
@@ -39,6 +29,10 @@ class JsonFile
     const LAX_SCHEMA = 1;
     const STRICT_SCHEMA = 2;
 
+    const JSON_UNESCAPED_SLASHES = 64;
+    const JSON_PRETTY_PRINT = 128;
+    const JSON_UNESCAPED_UNICODE = 256;
+
     private $path;
 
     /**
@@ -108,7 +102,7 @@ class JsonFile
                 );
             }
         }
-        file_put_contents($this->path, static::encode($hash, $options). ($options & JSON_PRETTY_PRINT ? "\n" : ''));
+        file_put_contents($this->path, static::encode($hash, $options). ($options & self::JSON_PRETTY_PRINT ? "\n" : ''));
     }
 
     /**
@@ -170,9 +164,9 @@ class JsonFile
 
         $json = json_encode($data);
 
-        $prettyPrint = (Boolean) ($options & JSON_PRETTY_PRINT);
-        $unescapeUnicode = (Boolean) ($options & JSON_UNESCAPED_UNICODE);
-        $unescapeSlashes = (Boolean) ($options & JSON_UNESCAPED_SLASHES);
+        $prettyPrint = (Boolean) ($options & self::JSON_PRETTY_PRINT);
+        $unescapeUnicode = (Boolean) ($options & self::JSON_UNESCAPED_UNICODE);
+        $unescapeSlashes = (Boolean) ($options & self::JSON_UNESCAPED_SLASHES);
 
         if (!$prettyPrint && !$unescapeUnicode && !$unescapeSlashes) {
             return $json;
diff --git a/tests/Composer/Test/Json/JsonFileTest.php b/tests/Composer/Test/Json/JsonFileTest.php
index 3090a450f..4e30928cb 100644
--- a/tests/Composer/Test/Json/JsonFileTest.php
+++ b/tests/Composer/Test/Json/JsonFileTest.php
@@ -140,9 +140,10 @@ class JsonFileTest extends \PHPUnit_Framework_TestCase
 
     public function testUnicode()
     {
-        if (!function_exists('mb_convert_encoding')) {
+        if (!function_exists('mb_convert_encoding') && version_compare(PHP_VERSION, '5.4', '<')) {
             $this->markTestSkipped('Test requires the mbstring extension');
         }
+
         $data = array("Žluťoučký \" kůň" => "úpěl ďábelské ódy za €");
         $json = '{
     "Žluťoučký \" kůň": "úpěl ďábelské ódy za €"
@@ -151,14 +152,23 @@ class JsonFileTest extends \PHPUnit_Framework_TestCase
         $this->assertJsonFormat($json, $data);
     }
 
-    public function testEscapedSlashes()
+    public function testOnlyUnicode()
     {
-        if (!function_exists('mb_convert_encoding')) {
+        if (!function_exists('mb_convert_encoding') && version_compare(PHP_VERSION, '5.4', '<')) {
             $this->markTestSkipped('Test requires the mbstring extension');
         }
-        $data = "\\/fooƌ";
 
-        $this->assertJsonFormat('"\\\\\\/fooƌ"', $data, JSON_UNESCAPED_UNICODE);
+        $data = "\\/ƌ";
+
+        $this->assertJsonFormat('"\\\\\\/ƌ"', $data, JsonFile::JSON_UNESCAPED_UNICODE);
+    }
+
+    public function testEscapedSlashes()
+    {
+
+        $data = "\\/foo";
+
+        $this->assertJsonFormat('"\\\\\\/foo"', $data, 0);
     }
 
     public function testEscapedUnicode()